I am working on a new Glen-L 14 build. I am a total amateur and am using this to build my knowledge base with wood working. In terms of employment I have always been a student or worked in education to some degree so this is nowhere near my wheel house but I saw the boats online and could never get the itch to build one out of my system. I have the plans and much of the lumber necessary. The fasteners and epoxy are on the way from Glen-L now. I am having trouble sourcing the plywood though. None of the distributors for marine or exterior grade plywood seem to be able to fill an order for sheets over 10'. I was wondering if this was a problem that seems to be unique to me or if this is a more common issue with a potential fix.

Sheets of plywood over 8 feet long are quite hard to come by. I only know of one place in Boston, MA that sells pieces up to 16 feet and they do it by joining two 8 foot pieces. You can order some up to 10 feet, but most designs here use longer sheets for planking. Best bet is to join sheets to get the length you need. There are two basic methods for making longer sheets, you can butt them together with glue and screws with a backing piece of plywood or you can make a scarf joint and glue them together. Look at this link here: http://www.glen-l.com/wood-plywood/scarf-butt.html
If you don't already have it, your best bet is to get a copy of the Glen-L book "Boatbuilding with Plywood" as it has all the info you want for materials and techniques to build your boat, it is highly recommended and the book nearly everyone here has: http://www.boatdesigns.com/Boatbuilding ... fo/12-430/

Murphy's Law: Anything that can go wrong, will go wrong.
Griffin's Law: Murphy was an optimist.

James, the beauty of these boats is that you can use a combination of the time it takes to build them, the information you get from this forum, and your own ability to learn to get one of these boats built. Nearly everyone here started off as a total amateur in this endeavor. Please keep us posted.

The plans should show something about where the plywood joins and is placed. Scarfing the joint is fine but to my knowledge a butt joint works well. You are going to lay fiberglass overtop. One of the things that happens with laying plywood on a curved surface is that it does not lay flat. The builder has to act quickly so that the epoxy does not harden before the screws are in. A 4 x 8 sheet has a lot of screws. It would be a bad experience to try and fit a larger sheet because it would twist and rock, it won't line up square that's for sure. One of the things I had to do was drill all the screw holes first. I clamped the sheet to the boat and scribed all the battens from inside then drilled the holes and set the screws in the plywood before applying epoxy between the batten lines on the plywood. You have to use a power screwdriver to get this done before the epoxy hardens and you have to work fast on a 4 x 8 sheet. You will have to clamp the wood to the battens and then start driving the screws. One of the things that might help you is to lay the plywood on the grass for an hour first. The moisture in the ground and the sun on the other side makes it cup. Much easier then forcing a flat piece.
A butt joint is perhaps stronger than a scarf. The problem is the wood is so thin that the screws find it hard to bite into the backing wood. I ended up grinding the screw points off after putting in the butt plates because I needed to be sure the plate was pulled in tight. You might try using #6 screws instead of #8. Fitting the butt plates between the frames is a bother too but it makes a strong joint when finished.

Look at some of the Zip builds, which is also 14' long, to see how they skinned the sides. I think you'll get some ideas on how you want to tackle this task. A lot will depend on your skill level and what tools you have access to. Butt joints are don't require anything special, but scarf joints will need some kind of jig and most commonly a router.

I scarfed the sides on my Squirt, and they are as strong as the surrounding wood.

The plywood will not flex at the reinforcement like it will through the rest of the sheet. The result will be a flat spot at the reinforced butt joint. If you decide to do a reinforced butt joint, try to locate it in a relatively flat section of the boat.

I have scarfed plywood with success using just a large hand plane and a big jar of elbow grease...Scribe a line a distance of 8 X the sheet thickness and parallel to the scarf joint on both sheets. Stack the two sheets with the scarf edge of the top sheet on the scribe line of the bottom sheet. Clamp the sheets together and lay the plane across the edge of the two sheets. That's the scarf angle you are looking for. Plane off the edge until you have a smooth flat surface from the scarf edge on the bottom sheet, through the scarf edge on the top sheet to the scribe edge on the top sheet. Now flip the top sheet over end-for-end and glue it up.

Every joint has its own problems. To avoid the material flattening at the seams it is best to put in a larger lap than that which is regarded as a minimum dimension. My laps were a good 12". Then I could pull it down with lots of screws but the screws needed to go all the way through and protrude on the other side. When the glue was hard, the tips had to be ground down slowly as not to burn the wood.

The problem with a butt joint between battens is getting the material tight against the batten. I also found that if you put a large bevel on the edge of the butt plate it is easier to avoid the flattening effect.

Plywood is much easier to scarf than solid wood. I used a belt sander and use the laminations as guide lines. Marking off the termination point and then removing the wood so that the lamination lines are equal spaced and straight, then match up the mating piece. The problem is that the wood is probably 4ft wide and if this is done on the boat you can't pull the scarf together tight enough for some glues. It pulls up the same way as the butt joint. Laying out the joint on a couple of 2 x 6's can pull it down. What you end up with is a big piece of wood and laying out a glue line, clamping and screwing can be a challenge for one person.

A lot of things are possible with good technique. I was able to use one piece of ply where the plans called for two because I used alternative heat and moisture to cup the sheet. It came out well and on other builds, as has been pointed out, the seam is often visible. But not on mine.